Abnormal synthesis of dolichol-linked oligosaccharides in carbohydrate-deficient glycoprotein syndrome

Carbohydrate-deficient glycoprotein syndrome (CDGS) is a rare metabolic disorder presenting in infancy with severe neurologic involvement and variable multisystemic abnormalities. Diagnosis relies upon the detection of abnormal serum glycoprotein isoforms on isoelectric focusing (IEF) gels. Carbohydrate structural analyses were performed on the N-linked oligosaccharides of serum α1-antitrypsin (α-1AT) from two Danish children with classical type I CDGS. Following preparative gel electrophoresis of α-1AT isoforms, oligosaccharide charge and monosaccharide composition analyses revealed increased glycosylation heterogeneity in CDGS compared with normal α-1AT. CDGS α-1AT isoforms bore N-glycans co-migrating with monosialylated standards, while normal α-1AT oligosaccharides co-migrated with both mono- and disialylated standards. While the monosaccharide contents of normal α-1AT isoforms were relatively uniform, those of CDGS α-1AT isoforms varied widely, and many were relatively mannose enriched. The mannose-rich oligosaccharides of CDGS α-1AT were not typical oligomannose structures since they were not released by endo-β-N-acetylglucosaminidase H (endo H) digestion. Metabolic labelling of CDGS fibroblasts with [3H]mannose showed lower than normal intracellular total mannose, free mannose and phosphorylated mannose species, as well as diminished [3H]mannose incorporation into dolichol-linked and protein-linked oligosaccharides. In addition, the glycans liberated from CDGS dolichollinked oligosaccharides were significantly truncated compared with those from normal fibroblasts. These data suggest that our type I CDGS patients produce abnormal N-linked oligosaccharides due to unpaired biosynthesis of dolichol-oligosaccharide precursors.